The active catalyst beds of hydroprocessing reactors have to be protected from solids and dissolved contaminants that are present in the feedstock. Typical solids are mill scale, dirt, and debris left in piping during construction and turnarounds. Entrained and dissolved species that range from organometallic compounds (e.g. organic nickel, vanadium, arsenic species) to sodium and chloride salts are also problematic. The solids are generally dealt with by utilizing a guard bed at the reactor inlet that has layers of progressively smaller sized inert material with high void volumes to capture the different sizes of solids, sometimes called a graded bed. If organometallic species are present, the grading material can also be composed of either porous or active catalyst to entrain and/or react with the offending species.
In the Fischer-Tropsch slurry reactor process, finely divided catalyst is suspended in a molten wax (e.g., predominantly paraffinic hydrocarbon) by bubbling synthesis gas through the reactor. The unique reaction conditions experienced in slurry bubble column processes are extremely harsh. The slurry reactor process causes catalyst attrition products, also referred to as contaminants, to be produced and get passed on in the product stream. The hydrocarbon reaction products are recovered in the overhead stream and from a slurry discharged from the reactor. The contaminants concentrate in the wax fraction that goes to downstream upgrading processes. The downstream upgrading processes are operated at hydroprocessing conditions which are typically between about 300° F. and 850° F. catalyst temperature, between about 100 psig and 3500 psig hydrogen partial pressure and typically employ liquid hourly space velocities (LHSV) between about 0.25 hr−1 and 5.0 hr−1. These catalyst attrition products may still be reactive and detrimental to those upgrading processes, reducing efficiency and causing shut downs. Thus, catalyst attrition losses in slurry bubble column processes can be problematic for hydroprocessing conditions.
The FT catalyst contaminants are generally submicron, which are not readily removed by conventional filters and stay in the feed until they reach the downstream upgrading processes, such as, a hydrocracker reactor. Guard beds have been historically used to capture catalyst fines, trap piping debris (e.g., mill scale, valve packing, etc.) and organometallic contaminants. Traditional guard bed applications accommodate increasing feed solids and/or contaminants loadings by increasing the guard bed depth, volume or packing void volume, or combinations thereof. Traditional guard beds are not designed to capture submicron particulates since typical feed contaminants tend to pass completely through subsequent reactor beds. However, in the case of the present invention, FT contaminants behave differently and hence need a new approach to effectively remove the submicron particulates.
A characteristic of FT catalyst contaminants is their propensity to form agglomerates in the catalyst beds of the hydroprocessing reactors. The agglomerates range from fairly stable to very fragile—the fragility indicated by its ability to waft in air upon disturbing the agglomerates. The FT agglomerates form in the interstitial spaces between particles (packing) and cause the packed bed to bridge (sometimes referred to as “plugging”) with increasing differential pressure being the result. The consequence of increasing differential pressure is the shortening of the run length for a given catalyst load which results in less production of products per annum.
When a hydroprocessing reactor experiences a high pressure drop associated with plugging, circulating a low viscosity diesel (or sometimes just recycle gas) through the unit can temporarily reduce the pressure drop when the wax feed is restarted. The pressure drop usually rises more rapidly with each successive attempt. It has been theorized that the change in flow regimes disturbs the bed and allows some of the agglomerates to redistribute themselves deeper into the bed.
Another unique feature of FT contaminants is the fact that they can form significant amounts of methane at hydrocracker operating conditions. Typical organometallic contaminants present in petroleum fractions do not produce methane at hydroprocessing conditions. It is believed that the cobalt present in the FT contaminants is responsible because of its methanating tendencies in the absence of hydrogen sulfide.
Another phenomenon that has been observed is exotherms in catalyst beds attributed to FT catalyst contaminants. Exotherms can occur at catalyst temperatures as low as ˜700° F. No exotherms have been experienced at hydrotreating temperatures (450-550° F.). Data to relate exotherm potential to FT catalyst fines concentration does suggest that higher concentrations of FT catalyst contaminants promotes instability.
Fischer-Tropsch catalyst typically employ a support material, primary active metal component and promoters. The support material can be alumina, titania, silica or combinations thereof. The metal component is traditionally cobalt, iron, ruthenium, platinum or nickel. Promoters are trace amounts of metal salts which promote certain reactions over others. FT catalyst contaminants that manage to get into the hydrocracker have a strong tendency to agglomerate. It is theorized that the combination of two-phase flow, the presence of hydrogen, and the low viscosity of the fluid at high temperatures promotes agglomeration of the submicron particles.
The plugging of the catalyst bed reduces operating runs, increases turnaround frequency and operating costs, and decreases plant efficiency. Additionally, methane production from FT liquids processing is undesirable. As demand for petroleum products increase, plant efficiency must be improved. Therefore, a method that removes solid particles from hydroprocessing feeds is needed.